Digital protection systems are used, for example, in power systems to monitor the voltage and current signals provided by electrical generators. Such digital protection systems may be embodied in a generator protection unit (GPU). A GPU estimates voltage and current phasors based upon the voltage and current signals and from those phasors detect various fault conditions requiring system shut down. The phasors are also used for metering functions performed by remote users and for other power system control functions. Abnormal conditions may additionally be detected through relatively large changes in the generator's operating frequency. Accordingly, an adaptive technique for tracking the generator frequency is important for both generating accurate voltage and current phasors and for monitoring fault conditions in the generator by detecting a significant change in the generator frequency.
FIG. 1 shows GPU 20 and generator 10. Voltage signals output from generator 10 (V.sub.out) are sensed by voltage sensor 12. Potential transformers, resistive dividers, or the like, may be employed as voltage sensors. The sensed voltage signals are output from voltage sensor 12 to GPU 20. It should be understood that generator 10 produces three-phase power, and accordingly, the voltage signals output from generator 10 may include V.sub.A, V.sub.B, and V.sub.C, or alternatively may include line-to-line voltages V.sub.AB, V.sub.BC, and V.sub.CA.
Current sensor 14 such as current transformers, may be used to sense current signals output from generator 10 (I.sub.out) to both GPU 20 and to the power system (not shown). If, for example, the current output to GPU 20 and to the power system are not equal then a fault condition may exist. The sensed currents, which may include each line current I.sub.A, I.sub.B, and I.sub.C, is provided as input to the GPU 20.
GPU 20 includes A/D converter 15, digital signal processor (DSP) 16, microprocessor 17, and external interface 18. A/D converter 15 samples the sensed voltage and current signals and provides voltage and current samples to DSP 16. DSP 16 may generate a voltage phasor each sampling interval by utilizing a discrete fourier transform (DFT). A DFT and system for implementing the transform are discussed in commonly assigned co-pending U.S. patent application Ser. No. 08/574,357, filed Dec. 18, 1995, entitled "System and Method for Phasor Estimation and Frequency Tracking in Digital Protection Systems," the contents of which are hereby incorporated by reference. The operating frequency of generator 10 can be tracked based on the generated phasors. Phasor data and frequency estimates are output from DSP 16 to microprocessor 17.
Microprocessor 17 uses the phasor and frequency data to detect faults in the power system. If a fault or malfunction is detected, microprocessor 17 outputs a signal through external interface 18 to a circuit breaker (not shown) causing the circuit breaker to open its current carrying contacts so that the system is effectively shut down.
Thus, a fault protection system as described above receives sensed voltage signals, and from these signals generates phasor and frequency data which is subsequently used to detect faults in the power system. As noted, typically such systems have three phase voltage inputs. If any one of the three signals are available, a fault detection system as described above could operate. However, if none of the input voltage signals are available, the protection system cannot operate. Thus, systems and methods of fault protection analogous to those described above present the problem of providing fault protection when the input sensed voltage signals are not available. Furthermore, because generators operate over a wide frequency range, particularly during startup or shutdown, any solution to the problem of loss of input voltage must also be capable of operating over a wide frequency range.